Gasoline engines on automotive vehicles have been controlled to operate close to a stoichiometric air-to-fuel ratio (AFR) so that carbon monoxide (CO), unburned or partially burned hydrocarbons (HC), and nitrogen oxides (NOX) in the exhaust stream can simultaneously be converted to carbon dioxide (CO2), nitrogen (N2), and water when passed in contact with a suitable platinum group metal (PGM) catalyst. In this mode of engine operation, the catalyst is characterized as a three-way-catalyst (TWC). With an ever increasing need for higher fuel economy there is current interest in operating gasoline engines, such as spark-ignition direct-injection (SIDI) engines, at AFR values that are relatively lean of the stoichiometric ratio (i.e., fuel-lean). The direct injection of gasoline into each engine cylinder allows a combustible air-fuel mixture to be formed near the spark plug for initiating combustion with leaner mixtures elsewhere in the combustion chamber. In some limited periods of engine operation, the AFR may be slightly rich of the stoichiometric ratio, but for most periods of engine operation the engine is operated fuel-lean to maximize fuel efficiency.
In comparison to stoichiometric AFR engine operation, fuel-lean engine operation reduces the amount of CO and HC in the exhaust, but increases the amount of NOX, which must be removed. The exhaust from such lean burn engines is still passed through a PGM catalyst-containing flow-through reactor to oxidize much of the CO and HC to CO2 and water. But then a reductant material is added to the exhaust stream that is reactive with NOX to convert it to N2 and water. The reductant material-containing exhaust is further passed through a reduction catalyst-containing flow-through reactor to promote the reduction of NOX to N2 and water. Since the added reductant material and the reduction catalyst must work together in treatment of the exhaust, this practice is called a selective catalytic reduction (SCR) with respect to the removal of NOX.
In a known SCR practice (especially for diesel engines), it is common to store an aqueous solution of urea on the vehicle and to inject a controlled amount of urea solution, as needed, into the exhaust stream. The urea quickly decomposes into ammonia (and carbon dioxide), and the ammonia serves as the reductant material for the NOX reduction reaction. The flowing stream is then passed over a suitable reduction catalyst, such as a particulate copper-exchanged zeolite and/or an iron exchanged zeolite material. This method of NOX reduction is commonly referred to as an NH3-selective catalytic reduction (SCR) of NOX.
The on-vehicle storage of a reductant material, such as urea, and its managed injection into the flowing exhaust from the engine permits flexibility in the management of lean-burn engine operation. The NOX content of the exhaust may be measured with a NOX sensor, and the addition of the reductant material may be computer-controlled in response to the output of the NOX sensor. But the vehicle operator must continually replenish the supply of urea solution, and must also keep the solution from freezing. Ammonia is a suitable reductant for NOX in a vehicle exhaust stream, but there would be a benefit if the reductant or its precursor did not have to be separately stored on the vehicle.
U.S. Patent Application Publication No. 2010/0107605 (the “'605 application”), titled “Passive Ammonia-Selective Catalytic Reduction for NOX Control in Internal Combustion Engines,” is assigned to the assignee of this invention and discloses a method of passively generating NH3 in an exhaust stream of a multi-cylinder, spark ignition, direct fuel injection, four stroke, gasoline engine that is primarily operated in a fuel-lean mode. But there are periods during which the “lean-burn” engine operates close to a stoichiometric AFR, or slightly fuel-rich of the stoichiometric ratio. Such periods may include, for example, engine-idle modes and vehicle acceleration modes of operation. The inventors in the '605 application recognized that oxygen-depleted engine exhaust contained NOX, CO, and hydrogen in sufficient quantities and proportions to form ammonia as the exhaust flowed through the PGM flow-through reactor close-coupled to the exhaust manifold of the engine. The inventors further recognized and disclosed practices for utilization of this passively-generated ammonia to reduce NOX in a NH3-SCR catalyst-containing flow-through reactor located downstream of the PGM reactor in the flow of the exhaust. The inventors recognized that, rather than adding urea solution to the exhaust, the passively-generated ammonia may eliminate the need for urea storage and injection.
The downstream NH3-SCR catalyst serves to temporarily store the passively-generated NH3 during fuel-rich operation. During subsequent periods of fuel-lean engine operation, the NH3-SCR catalyst effectively converts NOX in the exhaust stream to N2 and water using the stored NH3. But, this passive NH3 generation method requires the engine's modes of operation to be efficiently managed to provide enough NH3 to the NH3-SCR catalyst during fuel-rich operation so that a suitable supply of NH3 is available on the NH3-SCR catalyst during fuel-lean operation to remove NOX from the exhaust.
It is recognized that NH3 generation on a PGM catalyst is enhanced when the AFR of the engine's combustible mixture is about 14 to 14.2 during fuel-rich operation. However, when the engine is operating in this fuel-rich range, the PGM catalyst may not then serve to effectively oxidize the CO in the exhaust due to the temporarily diminished supply of oxygen. And the CO is not likely to be oxidized in the NH3 selective reduction reactor either.
One method of removing CO from the exhaust stream resulting from these fuel-rich periods includes passing the exhaust stream in contact with a second oxidation catalyst and with an auxiliary air injection, downstream of the PGM catalyst to promote the oxidation of residual CO in the exhaust to CO2 and water. However, this oxidation catalyst must be able to oxidize the residual CO without also oxidizing the NH3 which is to be used by the downstream reduction catalyst-containing flow-through reactor.